1. Field of the Invention
The present invention relates to an image forming apparatus capable of forming images on both front and rear sides of a sheet.
2. Description of the Related Art
Hitherto, in some of image forming apparatuses such as copying machines and page printers, after reversing a sheet having an image formed on one (front) side, the sheet is transported again to an image forming section to form an image on the other (rear) side. Such an image forming apparatus includes a duplex feed unit for reversing a sheet having an image formed on one side, and then transporting the sheet again to the image forming section. That duplex image formation, however, has a problem in that the efficiency is reduced in a continuous image forming mode because a sheet is circulated. The following techniques have been proposed as measures for overcoming the problem.
For example, a duplex feed unit disclosed in Japanese Patent Laid-Open No. 58-182655 includes a duplex copying aid means that comprises a sheet ejection section, a switching gate, a switchback section, a return section, and a reversing section. When copying an image on the rear side of a sheet on the front side of which an image has been copied, the duplex copying aid means increases a copy return speed so that the sheet having finished copying of an image on the front side more quickly reaches a predetermined return position from which copying of an image on the rear side can start. As a result, the efficiency of duplex copying can be increased.
Also, a duplex feed unit disclosed in Japanese Patent Laid-Open No. 62-161641 includes a feed control means for adjusting the interval in supply of sheets, on each of which images are to be formed on duplexs or in a superimposed manner, depending on feed conditions of the sheets or an external means. The feed control means operates to prevent a reduction of the maximum number of sheets printable per minute in a continuous printing mode.
Further, the duplex feed unit disclosed in Japanese Patent Laid-Open No. 6-35265 includes two stages of sheet reversing routes each having a reversing feed path. With this unit, continuously transported sheets can be consistently supplied again without stopping the sheets, and hence duplex image formation can be performed at a higher speed.
Moreover, in the duplex feed unit disclosed in Japanese Patent Laid-Open No. 2000-143103, a plurality of branched reversing sections are provided in a re-feed path, and sheets successively transported at a predetermined interval are introduced to the respective reversing sections and reversed therein. Then, the sheets are transported again to an image forming section at a sheet interval narrower than the predetermined interval, whereby a speed of duplex image formation can be increased.
However, the image forming apparatuses including those conventional duplex feed units have the problems given below.
In the apparatus including the duplex feed unit to increase the speed in transporting a sheet, on one side of which an image has been printed, to a reversing unit as disclosed in Japanese Patent Laid-Open No. 58-182655, or in the apparatus including the duplex feed unit to adjust the interval in supply of sheets depending on feed conditions of the sheets as disclosed in Japanese Patent Laid-Open No. 62-161641, a great improvement in processing speed cannot be expected even though a slight increase in speed of the duplex image formation is expected.
Also, in the apparatus including the duplex feed unit provided with two stages of sheet reversing routes each having a reversing feed path as disclosed in Japanese Patent Laid-Open No. 6-35265, the apparatus size is increased and the feed path has a larger length. Hence, the sheet transport speed must be increased to raise the speed of the duplex image formation.
Further, in the apparatus including the duplex feed unit provided with a plurality of switchback paths as disclosed in Japanese Patent Laid-Open No. 2000-143103, when the number of sheets successively transported to the re-feed path exceeds three, the number of sheets transported within the apparatus is suppressed because the interval between the sheets transported to the re-feed path must be held longer than the time required for reversing the sheet. When the number of sheets is two or less, circulative feed for the duplex image formation cannot be performed with high efficiency, and hence the speed of the duplex image formation cannot be increased.
In view of the state of the art set forth above, it is an object of the present invention to provide an image forming apparatus in which images can be formed on both sides of a sheet at high speed without increasing the apparatus size.
To achieve the above object, the present invention provides an image forming apparatus adapted to reverse a sheet on one side of which an image has been formed in an image forming section, transport the sheet back to the image forming section, and form an image on a second side of the sheet, the apparatus comprising an inlet feed path for transporting a sheet, on one side of which an image has been formed; a switchback path for withdrawing the sheet transported from the inlet feed path; a plurality of reverse feed paths each branching from the switchback path at one of plural positions, each of the plurality of reverse feed paths transporting a sheet while reversing the sheet; and a re-feed path for transporting the reversed sheet from any of the plurality of reverse feed paths back to the image forming section.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.